The Maxwell Scenic Highway extends down the west side, between the Chisos
Mountains and Burro Mesa through mostly volcanic terrain all the way to Santa
Elena Canyon, where you find
sedimentary rock in
the form of
limestone. Named for Ross
A. Maxwell, a geologist and the Park's first administrator, it is replete with
remarkable views and even more remarkable geologic features. Our first stop is
a syncline exposed in the
Chisos Formation in the area just west of the high Chisos Mountains. (There has
been some nomenclature problems here. When I created the image these rocks were
called the Chisos Formation, but then I read where "formation" had been changed
to "group". A group contains two or more formations. However, the new USGS map
of the park (Scientific Investigations Map 3142, 2011) has reverted back
to "formation". If you are by any wild chance interested, you can check out the
stratigraphic units
nomenclature used by geologists with this link.)

A syncline is a
geologic fold where
beds of rock have been bent downwards near the center of the fold
relative to the sides. A fold in geology can be one of many different types,
but the two most common are syncline and
anticline, where an
anticline is folded up in the middle, rather than down. Anticlines and
synclines are divided into two halves, called "limbs", by an imaginary plane
called the axial plane (sort of like how the Earth is divided into two
hemispheres by the equatorial plane). The axial plane "cuts" the fold at where
the curvature is greatest. However, where the plane should be drawn for this
fold isn't clear as far as I'm concerned. I gave it a shot, however. Note the
horizontal line and imagine it is the surface of the ground you are walking on,
and the cross section of the syncline is not visible. How would you know there
was a synclinal fold beneath your feet? Two ways. If you can determine the dips
of the beds, note they dip toward the center, and find the same beds on both
sides where the dip changes, you have identified the fold. If you can't
determine the dips, because there are no outcrops, but you can identify
the beds by age, note the same older beds ("o") are on either side of the
youngest beds ("y"), once again you know the fold is there. In this case,
however, a fault, discussed below, might complicate things. It's almost never
easy for the poor geologist.

The new USGS map shows an anticline off to the left of the syncline, but it
was not clearly visible from the road. Note the gully running up the hill on
the right side. According to the USGS map, it marks a
fault that cuts
through the syncline. This fault is a zone where fractured rock makes the job
of erosive forces easier, hence the formation of the gully. One reference says
the rock has been faulted down on the right (south) side of the fault, but the
USGS map does not indicate which side is up and which is down, and from the road
it is not clear how the movement along the fault occurred. The rocks that have
been folded in this syncline are designated the undifferentiated "younger part"
of the Chisos Formation on the USGS map, meaning they have not been formally
divided up into smaller stratigraphic units. It is clear, however, that much of
what you see here consists of lithified (turned into rock) volcanic ash called
tuff. The thin, darker beds are
probably lava flows.

From Sotol Vista, a turnout and viewpoint about seven miles south of the
turnoff onto Maxwell Scenic Highway, you get the expansive view seen below.
Goat Mountain is on the left and Kit Mountain center right. Goat Mountain is
topped with the volcanic
rhyolite member of the
Burro Mesa Formation, whereas Kit Mountain sports this rock on its west and
east ends as shown in the photo. In between the rhyolite on Kit Mountain is the
volcanic Wasp Spring member
of the Burro Mesa Formation lying on top of another volcanic rock called
trachyte. Rhyolite is a
volcanic rock high in
silica that is the
chemical equivalent of
granite. The difference is
the magma that becomes granite never reaches the surface and so cools and
solidifies underground. Were it to reach the surface, it would be rhyolite.
The term "trachyte" covers a number of quartz-poor volcanic rock types that are
high in alkali metals such as sodium and potassium. If it solidifies underground
it is called syenite, a sort
of low-quartz granite seen in the park at, for example,
Grapevine Hills. Several
radiometric dates
place the age of this rhyolite at 29 million years. A sample of trachyte dated
at Goat Mountain gave an age of 30.3 million
years. The Wasp Spring member records a violent eruption or eruptions. Its
texture indicates flows of ash and broken-up rock as well as volcanic blocks and
"bombs", which are formed when lava is thrown into the air and solidifies while
in flight. Ages for this member come in at a little over 29 million years. You
will see these units close up on Goat Mountain farther down the road. In the
distance you can see the
1400-foot scarp marking the eastern edge of Mesa de Anguila and, just visible
in the scarp over Kit Mountain, is the notch that marks the position of Santa
Elena Canyon. From this distance the canyon seems puny.

One of the most interesting geological features along the Maxwell highway
is Goat Mountain. When I first wrote about Goat Mountain in this field trip,
there was some uncertainty about the setting in which the volcanic vent, marked
now by a volcanic plug on the west side of the mountain exposed by erosion, was
active. Geologists Maxwell (The Big Bend of the Rio
Grande) and Darwin Spearing (Roadside Geology of Texas), write that
the eruption occurred in a pre-existing canyon, whereas an interpretive
exhibit at the Goat Mountain turnout and William MacLeod (Big Bend
Vistas) claim the "canyon" is actually the result of the eruption itself.
Closer examination of the mountain appears to support the latter
interpretation, but nothing is said about this in the latest USGS work. For a
long time I just left the photo I had marked up with an interpretation of the
mountain's geology as it was, with the canyon's former surface indicated by a
white line. (I've heard that slothfulness is one of the seven deadly sins, but
I haven't bothered to check if that's true.)

Well, I've finally made the change and got rid of the (likely) offending
interpretative photo. Plus, new information has been incorporated into the
photos shown below. Also, the previous interpretative photo was taken in bad
light. Recently I have been able to get better photos and use the USGS map for
a more up-to-date take on the geology. Unfortunately, the photo of the whole
mountain didn't turn out, but I did get four decent photos, of the north side,
a close-up of the north side, the south side, and the vent.

The north side of Goat Mountain is a layer cake of volcanic rocks. The two
photos below are of that layer cake, with the second photo being a close-up.
The lowest layer is the undifferentiated Chisos Formation rocks mentioned
above. This designation includes rocks that span the entire stretch of time of
the younger Chisos Formation rocks, with ages between 45 and 33 million years
as indicated by a number
of radiometric dates. In other words these rocks clearly belong to the Chisos
Formation but don't belong to the recognized members of that formation. That
means they need further study to see if they can be differentiated into distinct
members (assuming that's possible). Hence the younger Chisos Formation rocks
that lie above and below the recognized members and have not been categorized
are designated with the catch-all "undifferentiated" term and labeled Tcy on
the USGS map.

The next layer is the dark rock of the Bee Mountain
Basalt member of the Chisos
Formation, which was erupted over a period stretching from 34 to 33 million
years ago. Basalt is an iron-rich volcanic rock with relatively less silica than
rhyolite. The basalt here has not been dated to my knowledge, but the Mule Ear
Spring Tuff just above it was dated to 33.6 million years. Above the tuff is
some more of the undifferentiated rock of the Chisos Formation... and that does
it for the
Eocene to early
Oligocene Chisos
Formation. The rocks farther upstairs belong to the Oligocene Burro Mesa
Formation, beginning with a 30.3 million-year-old trachyte member. This trachyte
had been previously mapped as the 33 million-year-old Tule Mountain Trachyte,
but newer work (Daniel P. Miggins et. al. in USGS Circular 1327) has
shown that it is younger. Above that are the Wasp Spring and Burro Mesa Rhyolite
members that have already been described. Only the Burro Mesa Rhyolite and the
Wasp Spring can be seen in the photo of the south side of Goat Mountain, since
the lower units are covered by debris. However, farther to the right of the
photo they show up again.

Cutting through the Chisos rocks is an intrusion of rhyolite, marking the
site of a volcanic vent. The intrusion represents some of the last of the
magma that didn't make it out, and now it's a
volcanic plug. The
eruption resulted in the Wasp Springs Tuff and,
on top of that, the Burro Mesa Rhyolite. Both units were apparently erupted
from a number of volcanic vents on the west side of the Chisos Mountains. The
ages of these two units are so close, it seems likely they were erupted rather
closely in time, one after the other.

The next picture is of one of the most famous landmarks in the park. From
a distance you would swear there was this gigantic animal crouching down
behind a ridge with just his ears and the top of his head showing. The ears
look very mule-like, hence the name "Mule Ear Peaks". They are actually the
eroded remnants of a pair of
dikes.

The rock
of these dikes is mapped as Wasp Spring by the USGS, implying that these dikes
used to be volcanic fissures that violently erupted at least some of the Wasp
Spring member of
the Burro Mesa Formation found in this area of the park. The thickest exposures
of the Wasp Spring are found to the north of Mule Ear Peaks, at the Burro Mesa
pouroff and along Blue Creek. The new rock feature here is the intrusive rock
marked in the photo, also assigned to the Burro Mesa Formation. It is not dated
at this location, but two samples near the Burro Mesa pouroff give ages of 29
million years. Also in the picture is trachyte, as found at Goat Mountain.

Taking a closer look at the "Ears", you can make out the Mule Ear Spring Tuff
and the Bee Mountain Basalt. When I first looked at this view, it appeared that
the small peaks to the left of the Ears were part of a dike that extended from
the left Ear toward the Chisos. However, I didn't recall any such feature on a
previous hike to Mule Ear Peaks. A perusal of the Cerro Castellan quandrangle
proved that this "dike" feature was merely a problem of perspective in the dry
desert air. The small peaks are actually closer to the camera than the Mule Ear
Peaks.

As you get closer to Mesa de Anguila (called Sierra Ponce on the Mexican
side), you drive by Cerro Castellan (below, looking southeast), an erosional
remnant that, due to its rather isolated position, stands out prominently.
Thanks to the USGS map, I can correct, I hope, the geology of this feature from
what I originally understood. (It
turns out the interpretive diagrams of Cerro Castellan in Roadside Geology of
Texas by Darwin Spearing and Big Bend Vistas by William MacLeod are
apparently in error. A different interpretation based on the USGS map and The
Big Bend of the Rio Grande by Ross A. Maxwell is much more compatible with
what you see here. Yet, due to the differences and a lack of discussion in the
pamphlet accompanying the USGS map, I'm still tentative about what I now show
for this mountain, but here goes.) The Burro Mesa Rhyolite occupies a much
larger portion of the mountain than I had shown previously, with the Wasp
Spring of the Burro Mesa Formation occupying less. These two units protect the
rocks below from erosion and form a "caprock". I identify the upper
unconformity, following
Maxwell, as being the boundary between the Chisos Formation below and the Burro
Mesa Formation above, although Maxwell thought what is now identified as the
Burro Mesa was the South Rim Formation. However, the identity
of the unit beneath the Wasp Spring is not clear to me.
Presumably, after Maxwell, it is the Chisos Formation and may possibly be the
Mule Ear Spring Tuff as I originally thought, but it is not mapped as such by
the USGS. (Although possibly the smallness of the outcrop meant it could not be
resolved on the map.) Below this unit is the Bee Mountain Basalt member of the
Chisos Formation and beds of ash and tuff of undifferentiated Chisos Formation
rocks. The rock units here have been dated. The Bee Mountain Basalt
comes in at 33.4 million years old and the Wasp Spring at 29.3 million years.
However, the age for the Burro Mesa Rhyolite, 29.5 million years,
is incompatible with the Wasp Spring - even when you take into account the
"error bars" in the dates - so one or the other must be wrong. I suspect it is
the age of the Burro Mesa Rhyolite sample dated at this location, since the
dates at other locations are younger. And, to reiterate, these two units were
probably erupted so close in time that it is difficult for radiometric dating
to distinguish them.

Below is a view of Cerro Castellan from the side, looking east. Under the
Burro Mesa Formation, as mentioned above, there is a time break, called an
unconformity, during
which erosion took place. It appears what you are looking at here is what is
left of one side of a valley into which the Burro Mesa Formation was
deposited. The unconformity may mark what used to be part of the valley
floor. However, there may be another unconformity here within the Chisos
Formation. My guess is that the Mule Ear Spring Tuff is
the yellowish sliver above the gray rocks, which show layering just above the
basalt. These might be considered undifferentiated rocks of the Chisos
Formation. I'm not taking bets, however. Finally, note
the talus (debris) from rock falls of the Burro Mesa Formation. This is a
reminder that erosion is still at work, and, geologically speaking, Cerro
Castellan is not long for this world. (But hopefully its geology will be
clarified before then!)

Continuing along Maxwell Scenic Highway toward the Rio Grande, you cross a
landscape carved into the volcanism of the past by erosive forces. You see
lava, tuff, ash, and, in the following image, a remarkably tiny volcanic plug
(or, neck), which marks a vent through which some of the 29-million-year-old
Burro Mesa Rhyolite was extruded.
The flow of lava through a tube-like opening in the earth created a texture
where the lava was in contact with the walls of the tube, making the tube look
for all the world like a petrified tree stump. It even has a "knot". No
wonder this is what many thought it was. However, the "stump" is composed of
solidified lava, and the knot is probably where some piece of rock caught up
in the flow later weathered away, creating a hole. Other such vents have been
located around Cerro Castellan.

As you approach the Rio Grande, Mesa de Anguila (actually, Sierra Ponce, as
it is known in Mexico) looms up before you, 1400
feet above the river. This side of the mesa is a fault scarp due to the
Terlingua fault - the rock across
the Rio has been uplifted 3000 feet relative to that on this side, forming the
western boundary of the down-faulted crustal block upon which much
of Big Bend National Park sits. The east side of the block is bounded by the
fault that created the Sierra del Carmen escarpment. As mentioned in other
episodes of this virtual field trip, the Chisos Mountains are high, not because
they have been lifted up, but because the land around them has been eroded
down. The Chisos owe their existence to the very resistant rock that caps the
mountains.

In the image below you see that another fault has cut the mesa escarpment,
down on the left and up on the right. Also, the rock layers are labeled. On top
is the massive Santa Elena
Limestone, over 700 feet
thick. Below it is
the Sue Peaks Formation, a blend of
shale,
marl, and limestone.
Much less resistant than either the Santa Elena above it or the Del Carmen
Limestone below it, it tends to form slopes rather than cliffs. The 150-foot
thick Del Carmen is relatively resistant like the Santa Elena and is a
ledge-forming unit. Beneath that is the relatively easily eroded Telephone
Canyon Formation, consisting of 25 feet of limestone and marl.

The Rio Grande (out of sight at the base of the escarpment) is continually
undercutting the cliffs such that from time to time they give way. The result
is a
landslide or
rockfall. The
collapse you see below appears to
be a bit of both. Although it appears to have been a somewhat chaotic collapse,
which would make it a rockfall, there are also preserved bedding planes and
an upright block of Del Carmen Limestone, which imply a more coherent
collapse, consistent with a landslide. And, possibly, more than one event
led to what is seen here.

The canyon which was almost invisible from Sotol Vista (and is invisible on
days of poor air quality), and even if visible appears to be a tiny chink in
Mesa de Anguila (which itself looks like a something you could mount with a
couple of steps), now appears as a huge narrow gash in the mesa, with sheer
walls that tower well over 1000 feet above you. The canyon has frustrated many
a traveler trying to move up the river. There is no trail that continues
all the way through the canyon. The cliffs of Mesa de Anguila (and Sierra
Ponce) are
insurmountable. The currents in the canyon are swift and difficult to fight.

You can, however, cross the (usually) dry bed of Terlingua Creek, up and
over a shoulder of rock, and down to a riverside trail that leads up the
canyon, gradually playing out. The canyon deserves its own
entry in this compendium of field trips, so
this will wrap it up for the Maxwell Scenic Highway.